SOME of Asia's most popular, and fragile, attractions are in danger of being overrun, as soaring tourist numbers threaten ancient infrastructure.

The faded 'No Climbing' signs are no match for the tourists jostling to capture that perfect shot of the sun setting over the temple of Angkor Wat, Cambodia's most famous attraction.

The view is stunning but the chaotic scene at Phnom Bakheng, also known as the sunset temple, is not exactly relaxing and it can be a struggle to even get a picture with no strangers in it.

Visitors to Angkor National Park topped 1.15 million in 2010, up 25 per cent on the previous year.

And at tourist spots across the region it's a similar story.

As the world recovers from the financial crisis and infrastructure in developing countries improves, the number of people drawn to Asia's cultural and natural riches has exploded.

While this brings in much-needed revenue, observers fear that the growth in tourism is putting unprecedented pressure on precious and often fragile World Heritage sites.

At Angkor Wat, the most impressive of the park's many temples, tourists are largely free to wander around the 12th-century complex, ignoring one-way signs and clambering over fallen stones.

Many lean against the ancient walls, while others trace delicate bas reliefs with their fingers.

"You start to notice a little bit of wear and tear and you're not sure if it's from centuries of use or if it's from lots of tourists," said Rona Soranno, 36, from California, after completing a tour of the temple's inner courtyard.

Her 33-year-old partner Marcus Welsh added: "On the one hand it's totally awesome that I am able to step on the stones and be close to so much history, but you have to wonder what it's going to look like years from now."

According to the Global Heritage Fund, a US-based non-profit organisation that works to protect heritage sites in developing countries, "Angkor is highly endangered from this lack of control."

"These ruins are 600-800 years old and need to be protected from tourism using standard routes, coverings, walkways and enforcement. This is not rocket science," said the group's executive director, Jeff Morgan.

The Apsara Authority, which oversees Angkor's upkeep, says it has taken steps to minimise harm to the buildings by roping off the most fragile structures, employing more than 270 tourist guards and diverting people away from the most crowded sites.

And twice a year the authority meets with UNESCO officials and foreign experts to "discuss protection efforts and challenges", said Ngeth Sothy, the group's vice-director of the Department of Angkor Tourist Development.

Further east, the Great Wall of China, one of the world's most magnificent structures, sees some 10 million visitors a year.

It too is feeling the strain from soaring tourism.

Parts of the wall, which was built over centuries and stretches for more than 8800km, are covered in graffiti. People have also been known to camp and hold raves at the wall, often leaving behind litter.

Most of what remains is in bad shape, according to William Lindesay, a Briton who has spent nearly a quarter-century working on wall conservation.

"Only 550km are in very good condition - that is, the wall has a structure, with towers still intact," he told AFP late last year.

Measures have been taken to limit the damage and new regulations now forbid any construction within 500 metres of the site. But Lindesay says this is just not good enough.

"It's a story of disappearing history," he said. "Once it's gone, it's gone."

That it is possible to limit the damage to ancient sites while welcoming large numbers of visitors is proved in Indonesia, where some two million people flock to the ninth-century Buddhist temple Borobudur each year, according to the site's head of conservation Marsis Sutopo.

"Tourists are not allowed to smoke in Borobudur temple. They are not allowed to wear hard sole shoes," Sutopo said of the strict measures at the complex.

"We have a routine conservation. This year, we will clean the drainage on the bottom of the temple. Every two years we have to report to UNESCO about our effort," he said.

It is not just the region's oldest structures that have become victims of their own success.

For years, the 17th-century Taj Mahal, India's white-marbled monument to love that attracts some three million tourists annually, was as much at risk from tourism as from pollution before steps were taken in the late 1990s to reduce the impact from vehicle traffic and nearby factories.

The building underwent a major facelift in 2007 but conservationists say it can be fully protected only if the number of visitors is restricted.

"Footfalls are indeed a major pressure on the monument and so access to the white platforms in the main mausoleum should be restricted through the levy of high entry fee," said prominent Agra conservationist Rajan Kishore.

Asia's natural treasures are also struggling to keep up with an ever-growing number of visitors.

Vietnam's Halong Bay with its striking limestone cliffs and emerald waters drew more than 2.3 million tourists in the first 10 months of last year - up about 114 percent from the same period a year earlier.

Many explore the area by boat and it is a constant struggle to keep them from throwing waste overboard, observers say.

"Halong Bay is in big trouble from solid waste destroying the environment," said Morgan.

Faced with a myriad of problems, the future might seem gloomy for some of Asia's best attractions but according to Morgan, "good management of heritage sites despite millions of visitors is possible".

"The big question is when will we treat our heritage sites with the funding and respect they deserve?"